Heretofore, automotive drivelines employ a constant-velocity joint for connecting a transmission shaft to another transmission shaft and transmitting rotational power to the axles. In recent years, there has been a growing need for lighter constant-velocity joints and hence smaller constant-velocity joints. The mechanical strength, durability, load capacity, etc. of constant-velocity joints are determined by the basic dimensions of various components of the constant-velocity joints. It is necessary to determine those basic dimensions for smaller constant-velocity joints while maintaining the mechanical strength, durability, load capacity, etc. at desired levels.
Japanese Laid-Open Patent Publication No. 2001-330051 discloses a fixed constant-velocity universal joint having an outer joint member, an inner joint member, eight torque transmission balls, and a retainer. According to a disclosed technical concept for establishing basic settings of the universal joint, the ratio Rw (=W/PCR) of the axial width (W) of the inner joint member to the length (PCR) of a line segment interconnecting the center of a guide groove defined in the inner joint member and the centers of the torque transmission ball is selected to be in the range of 0.69≦Rw≦0.84.
Japanese Laid-Open Patent Publication No. 2003-97590 reveals a fixed constant-velocity universal joint having an outer race, an inner race, six torque transmission balls, and a cage. It is also disclosed that, if it is assumed that a drive shaft has a diameter d, and the torque transmission balls have a diameter DB and a pitch circle diameter DP, then the ratio DB/d of the diameter DB to the diameter d is set to a value in the range from 0.65 to 0.72, and the ratio DP/DB of the pitch circle diameter DP to the diameter DB is set to a value in the range from 3.4 to 3.8.
The publication, “Universal Joint and Driveshaft Design Manual, Advances in Engineering Series No. 7” (United States), edited by Charles E. Cooney, Jr., 2nd edition, The Society of Automotive Engineers, Inc., 1991, pp. 145-149, (hereinafter referred to as “general document”) discloses a Rzeppa constant-velocity joint. The disclosed Rzeppa constant-velocity joint has an outer race and an inner race, both having ball grooves therein. The ball grooves in the outer race and the ball grooves in the inner race have respective centers located on joint shafts (a drive shaft and a driven shaft) at respective positions that are offset equal distances from the center of the joint.
When the Rzeppa constant-velocity joint is in operation, the ball grooves in the outer race and the ball grooves in the inner race move relative to each other to position the six balls retained by the cage on a constant-velocity plane or a bisecting angle plane at one-half of the joint angle formed between the joint shafts, keeping drive contact points on the constant-velocity plane at all times to provide constant-velocity transmission.
The general document describes that the ball groove funnel angle, which is an angle formed between a common normal to load-side contact points between the outer race ball grooves (guide grooves) and the balls, and a common normal to load-side contact points between the inner race ball grooves (guide grooves) and the balls, is approximately in the range from 15 degrees to 17 degrees. This angular range allows the Rzeppa constant-velocity joint to make smooth angulation without encountering friction locking when passing through the 0-degree joint angle.
The general document also discloses that the ball grooves, in general, have a cross-sectional shape (perpendicular to the joint shafts) as a circular arc or an elliptical arc, and the contact angle between the ball grooves having an elliptical arc cross section and the balls is in the range from 30 degrees to 45 degrees, with 45 degrees being commonly used.
Japanese Laid-Open Patent Publication No. 2003-4062 and Japanese Laid-Open Patent Publication No. 9-317784 show fixed constant-velocity universal joints each having an outer race, an inner race, eight balls, and a cage. The outer race has guide grooves (track grooves) having respective groove bottoms which include a curved region whose center is offset a distance (F) in one direction from the center of an inside-diameter surface of the outer race, and the inner race has guide grooves (track grooves) having respective groove bottoms which include a curved region whose center is also offset the distance (F) in the opposite direction from the center of an outside-diameter surface of the inner race.
It is revealed in Japanese Laid-Open Patent Publication No. 2003-4062 that the ratio R1 (=F/PCR) of the offset distance (F) to the length (PCR) of a line segment interconnecting the center of the guide grooves in the outer race or the center of the guide grooves in the inner race to the centers of the balls is selected to be in the range of 0.069≦R1≦0.121.
Japanese Laid-Open Patent Publication No. 9-317784 discloses that the ratio R1 (=F/PCR) of the offset distance (F) to the length (PCR) of a line segment interconnecting the center of the guide grooves in the outer race or the center of the guide grooves in the inner race to the centers of the balls is selected to be in the range of 0.069≦R1≦0.121, and the contact angle between the guide grooves and the balls is set to a value equal to or smaller than 37 degrees.
Japanese Laid-Open Patent Publication No. 2002-323061 discloses a fixed constant-velocity universal joint having an outer joint member, an inner joint member, eight torque transmission balls, and a retainer. The ball grooves (track grooves) in the outer joint member and the ball grooves (track grooves) in the inner joint member have respective centers located at respective positions that are offset equal distances axially in opposite directions. The PCD gap (the difference between the pitch circle diameter of the ball grooves in the outer joint member and the pitch circle diameter of the ball grooves in the inner joint member) in the ball tracks is in the range from 5 to 50 μm.
According to Japanese Laid-Open Patent Publication No. 2002-323061, the PCD gap selected in the range from 5 to 50 μm is effective to increase the durability of the fixed constant-velocity universal joint and stabilize variations in the service life thereof under high loads.
It is also disclosed in Japanese Laid-Open Patent Publication No. 2002-323061 that the radial gap between the outer joint member and the inner joint member is in the range from 20 to 100 μm, and the radial gap between the retainer and the inner joint member is also in the range from 20 to 100 μm.
As shown in FIG. 24 of the accompanying drawings, one conventional constant-velocity joint of the above type has an outer member (outer race) 1 having a plurality of curved guide grooves 1b axially defined in a spherical inside-diameter surface 1a, and an inner member (inner race) 2 having a plurality of curved guide grooves 2b axially defined in a spherical outside-diameter surface 2a and splines 2c on an inside-diameter surface thereof. The guide grooves 1b in the outer member 1 and the guide grooves 2b in the inner member 2 make up ball rolling grooves with torque transmission balls 3 disposed therein. The torque transmission balls 3 are retained in respective retaining windows 4a defined in a substantially ring-shaped retainer 4.
The mechanical strength of the joint at the time the outer member 1 and the inner member 2 are at an angle to each other is determined by the mechanical strength of the retainer 4. Therefore, in order to increase the mechanical strength of the joint at the time the outer member 1 and the inner member 2 are at an angle to each other, the mechanical strength of the retainer 4 needs to be increased.
The mechanical strength of the retainer 4 is increased when the cross-sectional area of the retainer 4 is increased. The cross-sectional area of the retainer 4 may be increased by a method (hereinafter referred to as “first method”) wherein the inner spherical diametrical dimension of the retainer 4 is reduced and the outer spherical diametrical dimension of the retainer 4 is increased to increase the wall thickness of the retainer 4, a method (hereinafter referred to as “second method”) wherein the cross-sectional area of a region of the retainer 4 which undergoes forces tending to push out the balls 3 when the joint is angulated, or a method (hereinafter referred to as “third method”) wherein the cross-sectional area of columns 4b positioned between the windows 4a of the retainer 4 is increased.
According to the first and second methods, however, the retainer 4 becomes heavy and has a greater width, and the balls 3 tend to bite into the guide grooves 1b, reducing the durability of the outer member 1. It is possible, therefore, that the wider retainer 4 may not be assembled properly into the outer member 1.
According to the third method, if the columns 4b are elongated to reduce the opening areas of the retaining windows 4a, the balls 3 are apt to contact the columns 4b and cannot be neatly assembled in the retainer 4. If the retaining windows 4a are too small, then the inner member 2 cannot easily be assembled into the retainer 4.
Japanese Laid-Open Patent Publication No. 2002-13544 discloses a constant-velocity universal joint having a retainer with round corners 4c in retaining windows or pockets 4a, wherein the ratio R/D of the radius R of curvature of the round corners 4c to the diameter D of the balls 3 is set in the range of 0.22≦R/D.
However, the fixed constant-velocity universal joint disclosed in Japanese Laid-Open Patent Publication No. 2001-330051 is made of up a large number of parts, is highly costly to manufacture, and is difficult to produce in practice.
The dimensional settings of the fixed constant-velocity universal joint disclosed in Japanese Laid-Open Patent Publication No. 2003-97590 serve to increase the mechanical strength of the cage (retainer) which retains the torque transmission balls, and do not contribute to a reduction in the size of the fixed constant-velocity universal joint.
Ball tracks which are defined by the ball grooves in the outer race and the ball grooves in the inner race are funnel-shaped, progressively spreading in the axial direction from the inner end toward an outer opening of the outer race. As the ball grooves in the outer race and the ball grooves in the inner race are offset equal distances from the joint center, the depths of the ball grooves in the outer and inner races are not uniform in the axial direction.
With the structure disclosed in the general document, since the depths of the ball grooves in the outer race and the ball grooves in the inner race are small, when the constant-velocity joint operates at large joint angles or under high loads, the contact ellipse of the balls protrudes from the ball grooves, tending to bring the balls onto shoulders (edges) of the ball grooves, causing the balls to crack or to wear down the shoulders (edges) of the ball grooves, resulting in a reduction in the durability of the constant-velocity joint. Furthermore, when the constant-velocity joint is placed under a high load, the positions where the ball grooves and the balls contact each other are brought closely to the end of the inner race, and the contact ellipse of the balls protrudes from the ball grooves, thus increasing the contact pressure imposed on the balls grooves by the balls.
According to Japanese Laid-Open Patent Publication No. 2003-4062 and Japanese Laid-Open Patent Publication No. 9-317784, it is disclosed that the ratio R1 (=F/PCR) of the offset distance (F) to the length (PCR) of a line segment interconnecting the center of the guide grooves in the outer race or the center of the guide grooves in the inner race to the centers of the balls is set to a certain value. If the diameter of the balls is reduced or the size of the constant-velocity joint itself is reduced, and the wall thickness of the retainer, which is the mechanically weakest component, is to be maintained, then the guide grooves in the outer race and the inner race are necessarily of an insufficient depth, and the shoulders of the guide grooves tend to be cracked or worn as described above.
Japanese Laid-Open Patent Publication No. 2002-323061 describes that a fixed constant-velocity universal joint having eight torque transmission balls and a fixed constant-velocity universal joint having six torque transmission balls have different basic structures, and the PCD gaps thereof are set to inherent values adequate for their respective structures. Nothing is disclosed or suggested in this publication with respect to settings, such as the PCD gap, etc., for a fixed constant-velocity universal joint having six torque transmission balls.
It is important, in the design of fixed constant-velocity universal joints of the above type, how to establish the PCD (pitch circle diameter) gap with respect to ball tracks that are defined by the confronting ball grooves defined in the outer and inner joint members. If the PCD gap is too small, then it will be difficult to assemble the balls into the ball tracks, and restraint forces applied to the balls will be too large to allow the balls to make smooth rolling movement. If the PCD gap is too large, then striking noise will be generated between the balls and the windows of the retainer and the vibration of the joint itself will increase.
According to Japanese Laid-Open Patent Publication No. 2002-13544, the ratio R/D of the radium R of curvature of the round corners in the pockets of the retainer (retainer windows) to the diameter D of the balls is set for the purpose of increasing the durability and the mechanical strength of the retainer. However, the above ratio setting is not effective enough to increase the mechanical strength of the retainer.
A major object of the present invention is to provide a constant-velocity joint wherein the surface pressure acting on guide grooves due to contact with balls is reduced for increased durability.
Another object of the present invention is to provide a constant-velocity joint wherein the shoulders of guide grooves are prevented from being cracked or worn for increased durability.
Still another object of the present invention is to provide a constant-velocity joint having six balls wherein various clearances and an offset distance of the retaining windows of a retainer are set to optimum values to reduce the surface pressures, which are directly related to the service life of the constant-velocity joint, acting between outer race guide grooves and the balls and between inner race guide grooves and the balls, for increased durability.
A further object of the present invention to provide a constant-velocity joint which is capable of being designed for various dimensional settings suitable for a small joint size while maintaining various characteristics, i.e., mechanical strength, durability, load capacity, etc., at desired levels.
Yet another object of the present invention is to provide a constant-velocity joint, which includes a retainer having desired mechanical strength, and which is capable of being assembled with increased efficiency.